(1) Field of the Invention
The invention relates to a process of producing cement, in which a powdered raw materials mixture is de-acidified in a calcinator and subsequently burned in a kiln and before being de-acidified is preheated by a heat exchange with exhaust gas streams from the kiln and from the calcinator, wherein the exhaust gas streams from the kiln and from the calcinator are conducted in respective parallel series of heat exchange stages and the powdered raw materials mixture is transferred in alternation from a heat exchange stage of one of said series to a heat exhange of the other of said series, and to a plant for carrying out that process.
(2) Description of the Prior Art
In order to improve the de-acidification in the calcinator by a reduction of the carbon dioxide partial pressure and to improve the preheating of the powdered raw materials mixture before it enters the calcinator, it is known from Opened German Application No. 29 31 590 to supply the calcinator, e.g., with the heated air which has been used to cool the cement clinker formed by the burning of the powdered raw materials mixture and to supply the exhaust gas from the kiln to the heat exchanger used to preheat the powdered raw materials mixture. The exhaust gas streams from the kiln and from the calcinator are conducted in respective parallel series of heat exchange stages and the stream of the powdered raw materials mixture is passed in alternation from a heat exchange stage of one to said series to a heat exchange stage of the other of said series before the mixture is supplied to the calcinator. These measures have the result that the exhaust gas from the kiln does not enter the calcinator so that a relatively low carbon dioxide content can be maintained during the calcinating step. Besides, owing to the flow of the exhaust gases in two streams fed from different sources, a very desirable utilization of the sensible waste heat of the exhaust gases is permitted and the powdered raw material mixture can be preheated to a higher temperature than in a process using only a single exhaust gas stream because the hot exhaust gas from the kiln enters the heat exchanger at a temperature which exceeds the inlet temperature of the exhaust gas from the calcinator. A disadvantage inherent in said known plants resides in that approximately equal temperatures of the two exhaust gas streams from the heat exchanger cannot be ensured unless the weight ratio of the two streams of the powdered raw materials mixture is carefully controlled. This requirement involves a higher structural expenditure and a poorer utilization of heat because under conditions equal in other respects the exit temperatures of the exhaust gas streams depend on the weight ratio of powdered raw materials mixture and exhaust gas. Besides, a separate blower is required for handling each exhaust gas streams or an oversize single blower must be provided because the exhaust gas streams must be expected to differ in temperature and pressure.